Thanks all,
Comments much appreciated, but I know all that, as my background is
maths. I was just wanting to see how Lilypond does it.
And of course after I posted about oldish pages I recall of course that
you have to do circles with Beziers in fonts, as the arc operator is not
available. The approximation is very excellent if you use the right
control points. If people stumble across this thread in the future, and
want to draw circles with Beziers, these are good pages:
http://whizkidtech.redprince.net/bezier/circle/
http://whizkidtech.redprince.net/bezier/circle/kappa/
Andrew